1. Field of the Invention
The present invention relates to a temperature controller adopted for electric blankets, electric carpets, electric floor heaters, etc.
2. Description of the Prior Art
A heating apparatus such as an electric blanket includes a blanket proper and a temperature controller which is connected to the blanket proper via a cable. The temperature controller controls the calorific value of a heater wire disposed in the blanket proper, thus controlling the temperature of the blanket.
FIG. 1 is a block diagram showing an electric blanket and a conventional temperature controller for controlling the temperature of the blanket.
In the figure, the temperature controller comprises a sensor temperature detecting circuit 1, a temperature setting circuit 2, a zero-cross signal generating circuit 3, a sampling circuit 4, a comparison circuit 5a, a driving circuit 5b and a thyristor 6. To control the temperature of an electric blanket 7, the value of a temperature detection signal outputted from a temperature sensor 8 disposed in the blanket 7 is compared with a set temperature set in the temperature setting circuit 2, and, according to a result of the comparison, a current to be supplied to a heater wire 9 buried in a cloth of the blanket 7 is controlled.
The constitution and operation of the conventional temperature controller will be described.
The sensor temperature detecting circuit 1 receives a temperature detection signal outputted from the temperature sensor 8 arranged in the electric blanket 7, and, according to the received signal, outputs a temperature detection signal Sl to the comparison circuit 5a.
The temperature setting circuit 2 supplies a temperature setting signal S2 corresponding to a predetermined temperature to the sampling circuit 4.
The zero-cross signal generating circuit 3 receives an AC voltage from a commercial AC source 10 which supplies electric power to the heater wire 9, and detects a zero-cross point of the AC voltage. After detecting the zero-cross point, the circuit 3 generates a zero-cross signal S3 and supplies it to the sampling circuit 4.
When no zero-cross signal S3 is supplied, the sampling circuit 4 latches its output terminal voltage, and, when the zero-cross signal S3 is supplied thereto, the sampling circuit 4 picks up the temperature setting signal S2 outputted from the temperature setting circuit 2 to supply the same to the comparison circuit 5a.
The comparison circuit 5a compares the temperature setting signal S2 outputted from the sampling circuit 4 with the temperature detection signal Sl outputted from the sensor temperature detecting circuit 1. If the signals have the relation of Sl &gt;S2, i.e., if the temperature of the electric blanket 7 is lower than that set in the temperature setting circuit 2, the comparison circuit 5a supplies a driving signal to the driving circuit 5b.
After receiving the driving signal, the driving circuit 5b supplies a trigger signal S4 to a gate of the thyristor 6. Due to the trigger signal S4, the thyristor 6 becomes conductive to flow a current to the heater wire 9 in the sequence of an end lOa of the commercial AC source 10, the heater wire 9, an anode of the thyristor 6, a cathode of the thyristor 6, and another end 10b of the commercial AC source 10, thus increasing the temperature of the blanket 7.
According to the conventional temperature controller described in the above, the thyristor 6 will not be triggered if the width of the zero-cross signal S3 outputted from the zero-cross signal generating circuit 3 is narrow or if a variation in the circuit operation causes the zero-cross signal 3 to be asynchronous with the AC voltage outputted from the commercial AC source 10.
To cope with this problem, the conventional temperature controller outputs the zero-cross signal S3 of relatively wide width Tl as shown in FIG. 2(B) from the zero-cross signal generating circuit 3 at the time when the AC voltage shown in FIG. 2(A) outputted from the commercial AC source 10 changes from negative to positive.
However, if the width of the zero-cross signal S3 is wide, a value of the temperature setting signal S2 outputted from the sampling circuit 4 may coincide with a value of the temperature detection signal Sl outputted from the sensor temperature detecting circuit 1 at the time of, for instance, to which is not the zero-cross point, as shown in FIG. 2(C).
Due to this, the driving circuit 5b outputs the trigger signal S4 shown in FIG. 2(D) to turn ON the thyristor 6 as shown in FIG. 2(E).
If the thyristor 6 is turned ON fairly long time after the zero-cross point, a current suddenly flows to the heater wire 9 as shown in FIG. 2(F) to generate a high-frequency noise which becomes an electromagnetic wave noise that badly influence radius and televisions.